Nucleation of intragranular cracks during low cycle fatigue is governed by various micro-structural phenomena. Complex dislocation arrangements and rearrangements have been identified in several experimental studies carried out on cyclically loaded steel specimens. Different dislocation structures, such as vein, ladder, and/or cell structures, have been observed. They are related to an inhomogeneous localization of plastic strain, which is mostly accommodated by the ladder structures of dislocations, also named persistent slip bands. These regions of intensive slip generate on the material surface intrusions and extrusions, called persistent slip markings. The emergence of this rough relief leads to the initiation of fatigue cracks and can be considered as the first sign of fatigue damage. For a better comprehension of crack nucleation in 316LN stainless steel, low cycle fatigue tests and numerical simulations were performed. Specimens of 316LN steel with polished shallow notches were cycled with constant loading amplitude. In situ observations with a long distance microscope and scanning electron microscopy and electron back scattered diffraction analyses were used to observe fatigue crack initiation. Persistent slip markings have been identified. In parallel, a three-dimensional finite element model of crystalline plasticity, named Cristal-ECP, has been developed in both ABAQUS™ and CAST3M™ finite element codes. The numerical studies performed on various polycrystalline aggregates of 316LN steel have shown a heterogeneous localization of strain in bands. For a more precise computation and to introduce a grain size effect, geometrically necessary dislocations directly related and computed with the lattice curvature have been introduced in Cristal-ECP. The first simulations have shown a real influence of the geometrically necessary dislocations on the hardening slope and more heterogeneous mechanical fields.